This invention relates to a drum brake device with a single service brake shoe-actuator, intended in particular to provide a braking force equivalent to that of duo-two leading (hereinafter xe2x80x9cD2Lxe2x80x9d) type drum brake with two service brake shoe-actuators.
A drum brake device with a single shoe-actuator capable of providing a braking force equivalent to that of a D2L type drum brake device is disclosed by this applicant in the Japanese Patent Laid-Open Provisional Publication No. 10-47396.
This conventional drum brake device is briefly explained with reference to FIG. 10. In this drum brake device, first anchors 29, 29 are positioned between adjacent ends 40a, 50a of a pair of brake shoes 40, 50, and second anchors 11, 11 incorporating shoe clearance adjustment devices are provided between the other ends 40b, 50b of the brake shoes 40, 50. Further, pivot levers 60, 60 are pivotally mounted on intermediate portions of the brake shoes 40, 50; adjacent ends 60a, 60a of the pivot levers 60, 60 operationally engage with pistons 23, 23 housed in a wheel cylinder 20, i.e., the shoe-actuator, operated by during the service brake actuation; and the other adjacent ends 60b, 60b of the pivot levers 60, 60 engage with the second anchors 11, 11.
Regarding the operation of the service brake in the conventional drum brake, when a brake pedal is pressurized the wheel cylinder 20; the pistons 23, 23 press the adjacent ends 60a, 60a of the pivot levers 60, 60; the pivot levers 60, 60 pivot and move with a point of abutment between the other adjacent ends 60b, 60b and the second anchors 11, 11. An outward force applied to the pivot levers 60, 60 is transmitted to the brake shoes 40, 50 via pivot sections 61, 61, and urge the linings 43, 53 to frictionally engage with a brake drum, not shown in the figure.
When the brake drum is rotating in the direction of arrow A, the brake shoes 40, 50 move along the rotational direction A of the brake drum; the shoe end 40b and shoe end 50a are supported by the anchors 11, 29 respectively, thereby generating a braking force.
When the brake drum is rotating in the direction of arrow B, the brake shoes 40, 50 move along the rotational direction B; the shoe end 50b and shoe end 40a are supported by the anchors 11, 29 respectively, thereby generating a braking force, too.
The above-described conventional drum brake device has the following limitations.
When the wheel cylinder 20, i.e., the shoe-actuator, pivots and moves the pivot levers 60, 60 with the point of abutment at the second anchors 11, 11 supporting the other adjacent ends 60b, 60b, the brake shoes 40, 50 move relative to the pivot section 61, 61, and frictionally engage with the rotating brake drum. Thereafter, the brake shoes 40, 50 generate the braking force by being abutted against and supported by either the first or the second anchor 11, 29. When the brake shoes 40, 50 move together with the brake drum, the other adjacent ends 60b, 60b of both pivot levers 60, 60 slide on the second anchors 11, 11 while supporting the operation force applied by the wheel cylinder 20, i.e., shoe-actuator. Therefore, the conventional drum brake has an unavoidable decrease in braking efficiency due to the resistance at the sliding portions and the rotational resistance at the pivot sections 61, 61.
The sliding resistance at the other adjacent ends 60b, 60b and the rotational resistance at the pivot sections 61, 61 are increased proportional to the operational force of the wheel cylinder 20, i.e., shoe-actuator, which leads to be a lower durability of the device due to wearing and fatigue in these sections.
During braking, a thrust force from the pivot levers 60, 60 acts on the second anchors 11, 11 incorporating the shoe clearance adjustment devices. If the automatic shoe clearance adjustment devices, that keep the constant clearances between the brake drum and linings 43, 43, is utilized as the shoe clearance adjustment ones, the thrust force affects to the automatic adjustment action as a variance factor, which tends to vary the clearances between the brake drum and the linings 43, 53.
It is an object of this invention to improve the braking efficiency and the durability of the drum brake device and to attain a stable automatic shoe clearance adjustment action by allowing the pivot section of the pivot lever to swing.
This invention is based on a drum brake device comprising: a pair of brake shoes positioned to face each other and to be frictionally engagable with an inner circumferential surface of a brake drum; a first anchor positioned between first adjacent ends of the brake shoes; a second anchor positioned between second adjacent ends of the brake shoes opposite the first adjacent ends; a pivot lever with an intermediate portion pivotally supported on a first one of the brake shoes; a shoe-actuator positioned adjacent to the first anchor and being disposed between one end of the pivot lever and a second one of brake shoes; and a strut positioned between and functionally engaged with a second end of the pivot lever and the second one of the brake shoes. This invention is characterized in that an application force of the shoe-actuator is transmitted to the first one of the brake shoes via a pivot section of the pivot lever and transmitted to the second one of the brake shoes directly from said shoe-actuator and indirectly via the strut.
When the brake shoes frictionally engage and interact with the brake drum during braking, the pivot lever is supported by the strut free to swing in order to smoothly move together with the brake shoes, thereby providing a D2L type drum brake device with high braking efficiency despite the use of a single shoe-actuator.
The pivot lever smoothly moves until it contacts with one of brake shoes supported by the anchor, which eliminates overload on the pivot and abutment sections thereof and provides a highly durable device.
When incorporating the automatic shoe clearance adjustment mechanism, no overload is encountered when adjusting. Therefore, operation of the delicate automatic shoe clearance adjustment device, which requires micro-adjustment, may be stabilized.
This invention enables the downsizing of the pivot lever and strut for realizing the drum brake with one shoe-actuator for the service brake, which reduces the cost of manufacturing and the overall weight of the device.
The parking brake mechanism of this invention can easily be installed.